Conventionally, a seat belt apparatus included in a vehicle such as an automobile prevents, in the case of an emergency such as at a collision of the vehicle, an occupant from jumping out of the seat by restraining the occupant with a seat belt. Such a seat belt apparatus includes a seat belt retractor that retracts a seat belt. In the seat belt retractor, the seat belt is retracted around a spool when not worn. When the seat belt is to be worn, the seat belt is withdrawn and is fastened around the occupant. In the case of the emergency mentioned above, a locking mechanism included in the seat belt retractor is activated and locks the rotation of the spool in a direction of seat belt withdrawal, whereby the withdrawal of the seat belt is prevented. Thus, the occupant is restrained by the seat belt in the case of an emergency.
As a locking mechanism included in such a conventional seat belt retractor, a locking mechanism of a frame locking type is known (see U.S. Pat. No. 4,796,918, for example). This locking mechanism includes a plurality of external teeth that are provided on a flange of a spool and a plurality of internal teeth that are fewer than the external teeth and are provided on a frame that supports the spool. When a deceleration that is larger than a specific value is experienced by the vehicle, the spool as a whole rotates about a pivot displaced from the position of a rotational shaft of the spool that is for normal usage, causing some of the external teeth to mesh with the internal teeth provided on the frame, whereby the rotation of the spool is locked. In the locking mechanism of the frame locking type disclosed by U.S. Pat. No. 4,796,918, since the internal teeth are provided on the frame, no special members having such internal teeth are necessary. Therefore, the mechanism can have a relatively light weight. Hence, with this locking mechanism, a weight reduction of a vehicle is satisfactorily realized.
In such a locking mechanism of the frame locking type, when some of the external teeth of the spool come into mesh with the internal teeth of the frame, the external teeth of the spool and the internal teeth of the frame may come into contact with each other at the tips thereof. That is, the external teeth and the internal teeth may not mesh properly. If the external teeth are made to mesh with the internal teeth in such an improper state, a large load is applied to the spool via the seat belt with the inertia of the occupant. Therefore, while the plurality of internal teeth are not in mesh with corresponding ones of the plurality of external teeth, the spool abruptly moves in a direction of the load acting on the seat belt. Consequently, the meshing phase of one of the external teeth of the spool does not match with the meshing phase of the last one of the internal teeth of the frame, applying a large load to that internal tooth. Hence, the frame needs to have a high stiffness. This increases the manufacturing cost.
Accordingly, there is another proposal concerning a seat belt retractor in which the tips of external teeth each have a flat portion (see Japanese Patent No. 2950980, for example). Even if some of the external teeth are not properly in mesh with internal teeth, the flat portions of the external teeth first come into contact with the tips of the internal teeth and the spool as a whole then rotates about its pivot, whereby the plurality of internal teeth properly come into mesh with corresponding ones of the external teeth. Thus, the plurality of internal teeth and corresponding ones of the external teeth are more assuredly made to mesh with each other. In the seat belt retractor disclosed by Japanese Patent No. 2950980, any of the external teeth of the spool is prevented from engaging with only one of the internal teeth of the frame. Therefore, the load to be applied to the internal teeth can be reduced.